As Eldartford correctly pointed-out, any stereo amplifier operated in bridged-mono mode is a "fully balanced" amp. Therefore, the lowest-priced fully-balanced amp is the cheapest stereo amp you care to purchase...although this may not be the answer to your prayers ;-) Since you already have a balanced preamp, you have the ability to run a stereo amp in bridged-mono mode by running opposite-polarity signals to the 2 inputs and connecting your speakers across the (+) speaker terminals (this is true of conventional solid-state designs - tube amps will require an additional connection between the 2 sections of the output transformer, and you should consult the manufacturer for the correct wiring arrangement). This can work extremely well with an appropriate pair of amps but, in practice, not all amps are really appropriate. Generally speaking, the better an amp is at handling low-impedance / difficult loads, the better it will adapt to bridged-mono operation.
Best regards,
Steve McCormack
www.SMcAudio.com